Radiation image information read-out method and system

ABSTRACT

An image signal representing radiation image information on an object stored on a stimulable phosphor sheet is read out by scanning the stimulable phosphor sheet with a stimulating light beam and photoelectrically detecting light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector having a photoelectric surface. A variable transmittance medium whose transmittance to the light emitted from the stimulable phosphor sheet upon stimulation thereof is variable continuously or stepwise is inserted into the optical path of the light between the stimulable phosphor sheet and the photoelectric surface of the photodetector The transmittance of the variable transmittance medium is changed according to the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof so that the photoelectric surface is not saturated by an excessive amount of light impinging thereupon.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of and a system for readingout radiation image information stored on a stimulable phosphor sheet inwhich the stimulable phosphor sheet is exposed to stimulating rays,thereby causing it to emit light in proportion to the amount of energystored thereon during its exposure to the radiation and the lightemitted by the stimulable phosphor sheet upon stimulation thereof isphotoelectrically detected and converted into an electric image signalrepresenting the radiation image information, and more particularly toan improvement for preventing saturation of a photodetector having aphotoelectric surface.

[0003] 2. Description of the Related Art

[0004] When certain kinds of phosphors are exposed to radiation such asX-rays, α-rays, β-rays, γ-rays, cathode rays or ultraviolet rays, theystore a part of the energy of the radiation. Then, when the phosphorwhich has been exposed to the radiation is exposed to stimulating rayssuch as visible light, light is emitted from the phosphor in proportionto the stored energy of the radiation. A phosphor exhibiting suchproperties is referred to as “a stimulable phosphor”. It has been knownto use stimulable phosphors in radiation image recording and reproducingsystems (sometimes referred to as “computed radiography”). Specifically,a radiation image of an object, such as a human body, is recorded on astimulable phosphor sheet (a recording medium provided with a layer ofthe stimulable phosphor). The stimulable phosphor sheet, on which theradiation image has been stored, is then exposed to stimulating rays,such as a laser beam, which cause it to emit light in proportion to theamount of energy stored thereon during its exposure to the radiation.The light emitted by the stimulable phosphor sheet, upon stimulationthereof, is photoelectrically detected and converted into an electricimage signal. The image signal is used for reproducing the radiationimage of the object as a visible image on a recording medium such aphotosensitive material or a display such as a CRT. See JapaneseUnexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395,56(1981)-11397 and the like.

[0005] The radiation image recording and reproducing system ispractically advantageous in that as compared with conventionalradiographies using silver halide film, an image can be recorded over anextremely wide radiation exposure range.

[0006] When reading out radiation image information from the stimulablephosphor sheet in the radiation image recording and reproducing system,for instance, a light beam such as a laser beam is caused totwo-dimensionally scan the stimulable phosphor sheet storing thereon aradiation image, and the light emitted from the stimulable phosphorsheet upon stimulation thereof is transmitted to a photodetector throughan optical guide having a light inlet end face extending along the mainscanning line. The photodetector photoelectrically detects in time thelight emitted from the stimulable phosphor sheet upon stimulationthereof and an image signal made up of image signal components forrespective picture elements is obtained.

[0007] The photodetectors generally employed in such systems includethose utilizing an internal photoelectric effect, e.g., aphototransistor, a photodiode and the like, and those having anphotoelectric surface and utilizing a photoemissive effect on thephotoelectric surface, e.g., a photomultiplier. When those having aphotoelectric surface such as a photomultiplier are employed in theabove system, the following provision is generally made in order toimprove sensitivity of the photodetector.

[0008] That is, the photodetector of this type is generally providedwith an optical guide for collecting the light, emitted from thestimulable phosphor sheet upon stimulation thereof, to the photoelectricsurface. The provision involves increasing light collecting efficiencyto the photoelectric surface by improving the light collectingperformance of the optical guide and/or employment of a photoelectricsurface made of a material such as bialkali, e.g., Sb—K—Cs, which ishigh in quantum efficiency.

[0009] However, increasing the sensitivity of a photodetector with aphotoelectric surface gives rise to another problem that a saturationphenomenon, that the sensitivity of the photodetector deteriorates for awhile after detection of a large amount of light, is apt to occur at thephotoelectric surface and when a visible image is reproduced on thebasis of an image signal obtained from a saturated photodetector, aghost image appears and the image quality deteriorates.

[0010] The system may be employed in general industrial field as well asa medical field. In the medical field, the object is the human body andsince the irradiation dose does not greatly vary depending on the partwhose radiation image is to be taken, the amount of light emitted fromthe stimulable phosphor sheet upon stimulation thereof does not greatlyfluctuate.

[0011] To the contrast, in the general industrial field, where thesystem is used in non-destructive inspection of products such as a castiron block, an iron block and the like, various kinds of materials canbe the object. Accordingly, in order to obtain radiation imageinformation suitable for the respective kinds of materials, theirradiation dose varies depending on the kind of the object over a verywide range (two to three figures in terms of dose ratio).

[0012] Thus in the radiation image recording and reproducing system foruse in the general industrial field, the aforesaid saturation phenomenonis apt to occur, and accordingly, there has been a demand for aradiation image information read-out system in which the saturationphenomenon at the photodetector is suppressed with the sensitivity ofthe photodetector kept high.

[0013] The intensity of light emitted from the stimulable phosphor sheetupon stimulation thereof rapidly increases from initiation of exposureto the stimulating light and is maximized in a short time (e.g., inseveral ns) and then is gradually weakened, with the stimulable phosphorsheet keeping emitting light for a time unique to the phosphors on thestimulable phosphor sheet after termination of exposure to thestimulating light. The light emitted from the stimulable phosphor sheetafter termination of exposure to the stimulating light is generallyreferred to as “afterglow”. Accordingly when the stimulable phosphorsheet is scanned by the stimulating light and the light emitted from thestimulable phosphor sheet is photoelectrically read out in time series,the afterglow component of picture elements precedingly exposed to thestimulating light is read in addition to the light emitted from a givenpicture element upon stimulation thereof as the radiation imageinformation component for the picture element, which results inincomplete separation of image signal components for the pictureelements and deterioration in sharpness of the reproduced image.Accordingly, when the stimulable phosphors on the stimulable phosphorsheet exhibit long afterglow, the sharpness of the image deteriorates toan unacceptable level.

[0014] Such a phenomenon occurs substantially in proportion to theirradiation dose (radiation energy) of the stimulating light per unitarea of the stimulable phosphor sheet. Accordingly, for example, when apicture element where the amount of light emitted upon stimulation isrelatively small exists just behind a picture element where the amountof light emitted upon stimulation is relatively large, the afterglowcomponent from the large emission picture element is superposed on thelight emitted from the small emission picture element upon stimulationthereof, and the amount of light read out as that emitted from the smallemission picture element is increased by the amount of the afterglow.This reduces the difference between the image signal components for thelarge emission picture element and the small emission picture element ascompared with the actual difference therebetween, and accordingly thereproduced image deteriorates in contrast, i.e., the S/N ratio of theimage signal deteriorates.

[0015] In order to overcome this problem, there has been proposed amethod in which interference between image signal components for therespective picture elements due to response properties (e.g.,attenuation properties) of the light emitted from the picture elementsupon stimulation thereof is electrically corrected by adding, to each ofthe image signal components in time series obtained by scanning thestimulable phosphor sheet with the stimulating light, a differentialvalue of the image signal component. (See Japanese Patent PublicationNo. 2(1990)-15154.)

[0016] However this approach is disadvantageous in the following points.First this approach involves a large amount of calculation sinceinterference between image signal components for the respective pictureelements is corrected by calculation taking into account the responseproperties (e.g., attenuation properties) of the light emitted from thepicture elements upon stimulation thereof. Second though being able toavoid deterioration in sharpness and/or contrast of the radiation image,the approach cannot overcome the problem of deterioration in S/N ratio.

SUMMARY OF THE INVENTION

[0017] In view of the foregoing observations and description, a firstobject of the present invention is to provide a radiation imageinformation read-out system and method in which the saturationphenomenon at the photodetector is suppressed, thereby preventingappearance of a ghost image, with the sensitivity of the photodetectorkept high.

[0018] A second object of the present invention is to provide aradiation image information read-out system and method in which saidproblems caused by the afterglow inclusive of deterioration in S/N ratiocan be overcome in a simple manner and the radiation image informationcan be accurately read out at a high speed even if the scanning speed isincreased.

[0019] The first object of the present invention can be accomplished bya radiation image information read-out method and a radiation imageinformation read-out system in which the absolute value of the amount oflight impinging upon the photoelectric surface of the photodetector issuppressed by causing the light emitted from the stimulable phosphorsheet upon stimulation thereof to pass through a variable transmittancemedium whose transmittance is varied according to the amount of lightemitted from the stimulable phosphor sheet before impinging upon thephotoelectric surface or by changing the amount of stimulating lightaccording to the amount of light emitted from the stimulable phosphorsheet upon stimulation thereof.

[0020] That is, in accordance with a first aspect of the presentinvention, there is provided a radiation image information read-outmethod for obtaining an image signal representing radiation imageinformation on an object stored on a stimulable phosphor sheet byscanning the stimulable phosphor sheet with a stimulating light beam andphotoelectrically detecting light emitted from the stimulable phosphorsheet upon stimulation thereof by a photodetector having a photoelectricsurface, wherein the improvement comprises the steps of

[0021] inserting a variable transmittance medium whose transmittance tothe light emitted from the stimulable phosphor sheet upon stimulationthereof is variable continuously or stepwise into the optical path ofthe light between the stimulable phosphor sheet and the photoelectricsurface of the photodetector, and

[0022] changing the transmittance of the variable transmittance mediumaccording to the amount of light emitted from the stimulable phosphorsheet upon stimulation thereof so that the photoelectric surface is notsaturated by an excessive amount of light impinging thereupon.

[0023] The stimulating light beam may be visible light, a laser beam orthe like.

[0024] As the variable transmittance medium, an electrochromic elementwhose transmittance to light varies according to the direction ofcurrent, a NCAP type liquid crystal element or the like can be employedas those whose transmittance can be electrically changed. Further a NDfilter system in which a plurality of optical elements which aredifferent in transmittance are mechanically selectively inserted intosaid optical path can also be employed.

[0025] As a material for electrochromic element, amorphous WO₃(colorless to blue), IrO₂ (colorless to blue), viologen (colorless toblue), anthraquinone (colorless to red) or the like may be used, and maybe selected according to the color of light emitted from the stimulablephosphor sheet.

[0026] The NCAP (Nematic Curvilinear Aligned Phase) type liquid crystalelement is of encapsulated nematic liquid crystals. Generally the liquidcrystal has a rod-like molecule exhibiting electrooptic anisotropy. Themolecules are apt to orient along an oriented film, and in a normalstate where no electric field is applied to the liquid crystal element,the molecules orient inward of the capsules and incident light isscattered at the surface and inside of the liquid crystals depending onthe refractive properties of the crystals, whereby the liquid crystalelement becomes opaque. When an electric field is applied to the liquidcrystal element, the liquid crystals which are positive in dielectricanisotropy orient in the direction perpendicular to the surface of theelectrodes. When the liquid crystals are of the same refractive index asthe polymer which is outside the capsules and in which the liquidcrystals are dispersed, light travels straight without being scatteredand accordingly the liquid crystal element becomes transparent.

[0027] The variable transmittance medium may be disposed at the lightinlet end face of the optical guide or at the connection between theoptical guide and the photodetector when the optical guide is integratedwith the photodetector. Though not necessary, it is preferred that theoptical guide be integrated with the photodetector from the viewpoint ofsimplicity of handling. In the case of a ND filter system, the filtermust be moved to adjust the transmittance and accordingly it is notpreferred that the ND filter system is integrated with the photodetectoror the like to such an extent that the ND filter cannot make a relativemovement.

[0028] The degree by which the transmittance of the variabletransmittance medium is to be changed according to the amount of lightemitted from the stimulable phosphor sheet upon stimulation thereof maybe empirically determined and may be tabulated with respect to theamount of light so that the degree by which the transmittance of thevariable transmittance medium is to be changed can be known by referringto the table.

[0029] The amount of light emitted from the stimulable phosphor sheetupon stimulation thereof may be determined by actually exposing a partof stimulable phosphor sheet, which is limited not to affectreproduction of the radiation image, to the stimulating light anddetecting the amount of light emitted from the part or may be determinedby estimation based on the irradiation dose to which the stimulablephosphor sheet was exposed to the radiation upon taking the radiationimage and the energy of the stimulating light. Such estimation may beinput from the outside. In the case of the radiation image recording andreproducing system for industrial use, the kinds of objects are largerthan in the case of that for medical use. Accordingly, the radiographingmenu such as the material and the thickness of the object, theradiographing direction and the like is closely related to theirradiation dose and on the basis of this fact, the amount of lightemitted from the stimulable phosphor sheet upon stimulation thereof maybe determined according to the radiographing menu.

[0030] Also in the case where the amount of light emitted from thestimulable phosphor sheet upon stimulation thereof is determinedaccording to the radiographing menu, the degree by which thetransmittance of the variable transmittance medium is to be changed maybe empirically determined and may be tabulated in relation to theradiographing menu so that the degree by which the transmittance of thevariable transmittance medium is to be changed can be known by referringto the table on the basis of the menu.

[0031] The above description may also be applied to the followingsinventions.

[0032] In accordance with a second aspect of the present invention,there is provided a radiation image information read-out system forcarrying out the method in accordance with the first aspect of thepresent invention. That is, in accordance with the second aspect of thepresent invention, there is provided a radiation image informationread-out system for obtaining an image signal representing radiationimage information on an object stored on a stimulable phosphor sheet byscanning the stimulable phosphor sheet with a stimulating light beam andphotoelectrically detecting light emitted from the stimulable phosphorsheet upon stimulation thereof by a photodetector having a photoelectricsurface, wherein the improvement comprises

[0033] a variable transmittance medium which is variable continuously orstepwise in transmittance to the light emitted from the stimulablephosphor sheet upon stimulation thereof and is inserted into the opticalpath of the light between the stimulable phosphor sheet and thephotoelectric surface of the photodetector, and

[0034] a transmittance changing means which changes the transmittance ofthe variable transmittance medium according to the amount of lightemitted from the stimulable phosphor sheet upon stimulation thereof sothat the photoelectric surface is not saturated by an excessive amountof light impinging thereupon.

[0035] In accordance with a third aspect of the present invention, thereis provided a radiation image information read-out method for obtainingan image signal representing radiation image information on an objectstored on a stimulable phosphor sheet by scanning the stimulablephosphor sheet with a stimulating light beam and photoelectricallydetecting light emitted from the stimulable phosphor sheet uponstimulation thereof by a photodetector having a photoelectric surface,wherein the improvement comprises the step of

[0036] changing the amount of the stimulating light impinging upon thestimulable phosphor sheet continuously or stepwise according to theamount of light emitted from the stimulable phosphor sheet uponstimulation thereof so that the photoelectric surface is not saturatedby an excessive amount of light impinging thereupon.

[0037] The amount of the stimulating light impinging upon the stimulablephosphor sheet may be changed by directly controlling the source of thestimulating light to change the amount of stimulating light emitted fromthe source, or by inserting a variable transmittance medium whosetransmittance to the stimulating light is variable continuously orstepwise into the optical path of the stimulating light from thestimulating light source to the stimulable phosphor sheet and changingthe transmittance of the variable transmittance medium with the amountof stimulating light emitted from the source unchanged. The variabletransmittance medium may be an electrochromic element, a NCAP typeliquid crystal element, a ND filter system or the like. When the amountof the stimulating light impinging upon the stimulable phosphor sheet ischanged, the levels of the signal representing detection of the leadingend of the sheet (in the sub-scanning direction) and the signalrepresenting detection of the scanning starting point (in the mainscanning direction) are changed. Accordingly it is preferred that thegains of such signals be automatically adjusted.

[0038] The above description may also be applied to the followingsinventions.

[0039] In accordance with a fourth aspect of the present invention,there is provided a radiation image information read-out system forobtaining an image signal representing radiation image information on anobject stored on a stimulable phosphor sheet by scanning the stimulablephosphor sheet with a stimulating light beam and photoelectricallydetecting light emitted from the stimulable phosphor sheet uponstimulation thereof by a photodetector having a photoelectric surface,wherein the improvement comprises

[0040] a means for changing the amount of the stimulating lightimpinging upon the stimulable phosphor sheet continuously or stepwiseaccording to the amount of light emitted from the stimulable phosphorsheet upon stimulation thereof so that the photoelectric surface is notsaturated by an excessive amount of light impinging thereupon.

[0041] In the radiation image information read-out method and system ofthe first and second aspects of the present invention, the level of theamount of light impinging upon the photoelectric surface of thephotodetector is suppressed not to saturate the photoelectric surface bychanging the transmittance of the variable transmittance medium disposedin the optical path of the light emitted from the stimulable phosphorsheet upon stimulation thereof between the stimulable phosphor sheet andthe photoelectric surface according to an estimated or measured amountof the light emitted from the stimulable phosphor sheet upon stimulationthereof.

[0042] For example, when the amount of light is large, the transmittanceof the variable transmittance medium is reduced so that a smaller partof the light emitted from the stimulable phosphor sheet can reach thephotoelectric surface, whereby generation of image signal componentswhich can produce a ghost image due to saturation of the photoelectricsurface can be suppressed.

[0043] On the other hand, when the amount of light is small, thetransmittance of the variable transmittance medium is kept high so thatan image signal can be obtained at a high sensitivity and deteriorationin S/N ratio can be suppressed. In this case, since the amount of lightemitted from the stimulable phosphor sheet upon stimulation thereof isoriginally small, there is no fear that the photoelectric surface issaturated.

[0044] In the radiation image information read-out method and system ofthe third and fourth aspects of the present invention, the level of theamount of light impinging upon the photoelectric surface of thephotodetector is suppressed not to saturate the photoelectric surface bychanging the amount of stimulating light impinging upon the stimulablephosphor sheet according to an estimated or measured amount of the lightemitted from the stimulable phosphor sheet upon stimulation thereof.

[0045] For example, when the amount of light expected to be emitted fromthe stimulable phosphor sheet upon stimulation thereof is large, theamount of stimulating light impinging upon the stimulable phosphor sheetis reduced so that a smaller amount of light is emitted from thestimulable phosphor sheet, whereby the level of the amount of lightimpinging upon the photoelectric surface of the photodetector issuppressed and generation of image signal components which can produce aghost image due to saturation of the photoelectric surface can besuppressed.

[0046] On the other hand, when the amount of light expected to beemitted from the stimulable phosphor sheet upon stimulation thereof issmall, the amount of stimulating light is not reduced (if possible, maybe increased, for instance, by controlling the stimulating light source)and accordingly, the amount of light emitted from the stimulablephosphor sheet is not reduced, whereby an image signal can be obtainedat a high sensitivity and deterioration in S/N ratio can be suppressed.In this case, since the amount of light emitted from the stimulablephosphor sheet upon stimulation thereof is originally small, there is nofear that the photoelectric surface is saturated.

[0047] In the radiation image information read-out methods and systemsof first to fourth aspects of the present invention, when the amount oflight emitted from the stimulable phosphor sheet upon stimulationthereof is estimated on the basis of the radiographing menu, necessityof actually detecting the amount of light is eliminated and algorithmfor the system can be simplified.

[0048] In accordance with a fifth aspect of the present invention, thereis provided a radiation image information read-out method for obtainingan image signal representing radiation image information on an objectstored on a stimulable phosphor sheet by exposing the stimulablephosphor sheet to stimulating light and photoelectrically detectinglight emitted from the stimulable phosphor sheet upon stimulationthereof by a photodetector, wherein the improvement comprises the stepof

[0049] setting the sensitivity of the photodetector so that the amountof light emitted from a blank portion on the stimulable phosphor sheetbearing thereon no radiation image information of the object becomeslarger than that corresponding to the upper limit of the operable rangeof the photodetector, and

[0050] controlling the irradiation energy of the stimulating light sothat the level of a signal component obtained by the photodetector byreading the light emitted from the blank portion is minimized in therange higher than the upper limit of the read-out signal level rangecorresponding to the operable range of the photodetector.

[0051] The “blank portion” on the stimulable phosphor sheet bearingthereon no radiation image information of the object is a portion which,when the radiation image was recorded, was directly exposed to theradiation without passing through the object. Further “the operablerange of the photodetector” is a range of the amount of light in whichthe photodetector can correctly convert the amount of light into anelectric signal component.

[0052] The expression “controlling the irradiation energy of thestimulating light” means to control the effective energy of thestimulating light to which the stimulable phosphor sheet is exposed perunit area thereof. The irradiation energy of the stimulating light canbe controlled, for instance, by attenuating the stimulating light fromthe stimulating light source such as a laser by an acoustoopticmodulator or the like provided on the optical path of the stimulatinglight, or by, in the case where the stimulating light source is asemiconductor laser, controlling the input voltage to the semiconductorlaser, or by increasing the scanning speed of the stimulating lightbeam.

[0053] In the radiation image information read-out method of the fifthaspect, it is preferred that “preliminary read-out” be effected prior to“final read-out” and the sensitivity of the photodetector be set on thebasis of the image information obtained by the preliminary read-out.

[0054] As disclosed, for instance, in Japanese Unexamined PatentPublication Nos. 58(1983)-67241, 58(1983)-67243 and 58(1983)-83937, the“preliminary read-out” is a well-known technique in which brief of theradiation image information stored on a stimulable phosphor sheet isread out prior to the “final read-out” by exposing the stimulablephosphor sheet to stimulating light having a lower level than that usedin the final read-out and reading out light emitted from the stimulablephosphor sheet upon stimulation by the lower level stimulating light.

[0055] A plurality of levels of the sensitivity of the photodetector maybe registered in advance in relation to different radiographing menusand the sensitivity of the photodetector may be set to the sensitivitylevel corresponding to the radiographing menu for the radiation imageinformation to be read out.

[0056] The “radiographing menu” means the part of the object, the methodof radiographing and the like and includes, for instance, chestradiographing, head radiographing, angiography and the like.

[0057] In accordance with a sixth aspect of the present invention, thereis provided a radiation image information read-out system for obtainingan image signal representing radiation image information on an objectstored on a stimulable phosphor sheet comprising a stimulating lightprojecting means which exposes the stimulable phosphor sheet tostimulating light thereby causing the stimulable phosphor sheet to emitlight in proportion to the amount of energy stored thereon and aphotodetector which detects the light emitted from the stimulablephosphor sheet upon stimulation thereof, wherein the improvementcomprises

[0058] a sensitivity setting means which sets the sensitivity of thephotodetector so that the amount of light emitted from a blank portionon the stimulable phosphor sheet bearing thereon no radiation imageinformation of the object becomes larger than that corresponding to theupper limit of the operable range of the photodetector, and

[0059] a stimulating energy control means which controls the irradiationenergy of the stimulating light so that the level of a signal componentobtained by the photodetector by reading the light emitted from theblank portion is minimized in the range higher than the upper limit ofthe read-out signal level range corresponding to the operable range ofthe photodetector.

[0060] It is preferred that the radiation image information read-outsystem of the sixth aspect be provided with a preliminary read-out meanswhich effects preliminary read-out prior to final read-out of theradiation image information and the sensitivity setting means sets thesensitivity of the photodetector on the basis of the image informationobtained by the preliminary read-out.

[0061] Further it is preferred that the radiation image informationread-out system of the sixth aspect be provided with a means forregistering a plurality of levels of the sensitivity of thephotodetector in relation to different radiographing menus and thesensitivity setting means sets the sensitivity of the photodetector tothe sensitivity level corresponding to the radiographing menu for theradiation image information to be read out.

[0062] In the radiation image information read-out method and system ofthe fifth and sixth embodiments of the present invention, since thesensitivity of the photodetector is set in the aforesaid manner and theirradiation energy of the stimulating light is controlled in theaforesaid manner, the afterglow component of the light emitted from theblank portion is reduced with the light emitted from the portion otherthan the blank portion kept at the normal level. Since the afterglowcomponent itself is reduced, noise due to the afterglow component can bereduced and the S/N ratio of the read-out signal can be improved.Further since the light emitted from the portion other than the blankportion is kept at the normal level, the image information for theobject necessary for diagnosis can be obtained at a proper quality.

[0063] When the preliminary read-out is effected prior to the finalread-out and the sensitivity of the photodetector is set on the basis ofthe image information obtained by the preliminary read-out, setting ofread-out condition is facilitated. Further when the sensitivity of thephotodetector is set on the basis of the radiographing menu, setting ofread-out condition is also facilitated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064]FIG. 1 is a schematic view for illustrating the basic arrangementof a radiation image information read-out system,

[0065]FIG. 2 is a view showing an important part of a radiation imageinformation read-out system in accordance with a first embodiment of thepresent invention,

[0066]FIG. 3 is a view showing an important part of a radiation imageinformation read-out system in accordance with a second embodiment ofthe present invention,

[0067]FIG. 4 is a schematic view showing a radiation image informationread-out system in accordance with a third embodiment of the presentinvention,

[0068]FIG. 5 is a schematic view showing a radiation image informationread-out system in accordance with a fourth embodiment of the presentinvention,

[0069]FIG. 6 is a view for illustrating the operation of the preliminaryread-out means employed in the radiation image information read-outsystem in accordance with the fourth embodiment,

[0070]FIGS. 7A and 7B are views showing the relation between the amountof light emitted from the stimulable phosphor sheet and the level of thesignal output from the photodetector, and

[0071]FIGS. 8A to 8E are views for illustrating the method ofcontrolling the energy of the stimulating light.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] In FIG. 1, a radiation image information read-out system to whichthe present invention is to be applied comprises an endless belt 40which is driven by an electric motor (not shown) to convey a stimulablephosphor sheet 30 placed thereon in the direction of arrow Y (thesub-scanning direction), a laser 11 which is located above thestimulable phosphor sheet 30 and emits a stimulating laser beam L forstimulating the stimulable phosphor sheet, a rotating polygonal mirror12 which deflects the laser beam L, an electric motor 13 for rotatingthe polygonal mirror 12 and a scanning lens (fθ lens) 14 which focusesthe laser beam L, deflect by the polygonal mirror 12, on the stimulablephosphor sheet 30 and causes it to scan the stimulable phosphor sheet 30at a constant speed.

[0073] An optical guide 21 is located above the stimulable phosphorsheet 30 close to the scanning line of the laser beam L to collect lightM emitted from the upper surface of the stimulable phosphor sheet 30upon stimulation thereof by the laser beam L. A photomultiplier 22 isconnected to the optical guide 21 by way of a stimulating light cutfilter 23, which prevents the laser beam L to enter the photomultiplier22, and photoelectrically converts the collected light M to an analogimage signal y.

[0074] A logarithmic amplifier 24 is connected to the photomultiplier 22and outputs a logarithmic image signal by logarithmic conversion of theanalog image signal y. An A/D convertor 25 is connected to thelogarithmic amplifier 24 and digitizes the logarithmic image signal ginto a digital image signal S. The digital image signal S is output toan external image processing system.

[0075] The operation of the radiation image information read-out systemshown 1 will be briefly described hereinbelow.

[0076] A stimulable phosphor sheet 30 storing thereon radiation imageinformation is set on the endless belt 40 in a predetermined position.Then the stimulable phosphor sheet 30 is conveyed (sub-scanning) in thedirection of arrow Y by the endless belt 40.

[0077] The laser beam L emitted from the laser 11 is deflected by thepolygonal mirror 12 which is rotated at a high speed in the direction ofarrow by the motor 13 and is focused on the surface of the stimulablephosphor sheet 30 by the scanning lens 14 and is caused to scan thesurface of the stimulable phosphor sheet 30 at a constant speed in thedirection of arrow X (main scanning). Thus the stimulable phosphor sheet30 is exposed to the laser beam L over the entire area thereof.

[0078] The portion of the stimulable phosphor sheet 30 exposed to thelaser beam L emits light M in proportion to the amount of energy storedthereon.

[0079] The light M emitted from the exposed parts of the stimulablephosphor sheet 30 in sequence is guided to the photomultiplier 22 by theoptical guide 21. At this time also a part of the laser beam L, e.g., apart of the laser beam L reflected at the stimulable phosphor sheet 30,enters the optical guide 21.

[0080] The light M and the laser beam L entering the optical guide 21impinge upon the stimulating light cut filter 23 between the opticalguide 21 and the photomultiplier 22, and the laser beam L is cut by thefilter 23 while the light M passes through the filter 23 to impinge uponthe photomultiplier 22.

[0081] The photomultiplier 22 photoelectrically converts the light M toan analog image signal y corresponding to the amount P of the light Mimpinging upon the photomultiplier 22 and outputs the analog imagesignal y to the logarithmic amplifier 24.

[0082] The logarithmic amplifier 24 outputs a logarithmic image signal gby logarithmic conversion of the analog image signal y. The logarithmicimage signal g is input into the A/D convertor 25 and is sampled atpredetermined sampling intervals in synchronization with the scanning bythe laser beam L, thereby quantized into a digital image signal S madeup of image signal components for the respective picture elements.

[0083] A radiation image information read-out system in accordance witha first embodiment will be described with reference to FIG. 2,hereinbelow.

[0084] The radiation image information read-out system of thisembodiment mainly differs from that shown in FIG. 1 in that anelectrochromic element 51 is disposed between the stimulating light cutfiler 23 and the photomultiplier 22. The transmittance of theelectrochromic element 51 to the light M emitted from the stimulablephosphor sheet 30 upon stimulation thereof can be changed.

[0085] As the material for the electrochromic element 51, amorphous WO₃(colorless to blue), IrO₂ (colorless to blue), viologen (colorless toblue), anthraquinone (colorless to red) or the like may be used.

[0086] The radiation image information read-out system of thisembodiment further comprises a light amount determination means 53 whichdetermines the amount of light M which is expected to impinge upon thephotomultiplier 22 through estimation on the basis of the radiographingmenu for the stimulable phosphor sheet 30 input from the exterior, and atransmittance changing means 52 which changes the transmittance of theelectrochromic element 51 on the basis of the amount of light M which isexpected to impinge upon the photomultiplier 22 determined by the lightamount determination means 53 so that the photoelectric surface of thephotomultiplier 22 is not saturated by an excessive amount of lightimpinging thereupon.

[0087] The operation of the radiation image information read-out systemof this embodiment will be described hereinbelow.

[0088] The radiographing menu for the stimulable phosphor sheet 30 to beread out is first input into the light amount determination means 53.The radiographing menu includes the kind of the object, the material ofthe object, the shape of the object, the size of the object, thedirection of radiographing and the like and the irradiation dose of theradiations to which the object was exposed can be determined accordingto the radiographing menu. The light amount determination means 53determines the amount of radiation energy stored on the stimulablephosphor sheet 30 on the basis of the radiographing menu input andcalculates the amount of light M expected to be emitted from thestimulable phosphor sheet 30 upon stimulation thereof by the laser beamL on the basis of the amount of radiation energy stored on thestimulable phosphor sheet 30 and the stimulating energy of the laserbeam L.

[0089] The radiographing menu may be input into the light amountdetermination means 53 by reading information recorded on the stimulablephosphor sheet 30, e.g., by reading a bar code representing the menu byuse of a bar code reader, or by way of information on the radiographingmenu of the stimulable phosphor sheet associated with the ID informationfor the sheet 30 which is read out from a server computer andautomatically input into the light amount determination means 53.Otherwise the operator may manually input information on theradiographing menu to the light amount determination means 53.

[0090] The amount of light M expected to be emitted from the stimulablephosphor sheet 30 determined by the light amount determination means 53is input into the transmittance changing means 52 and the transmittancechanging means 52 changes the transmittance of the electrochromicelement 51 according to the amount of light M expected to be emittedfrom the stimulable phosphor sheet 30.

[0091] That is, when the amount of light M expected to be emitted fromthe stimulable phosphor sheet 30 is large, the transmittance of theelectrochromic element 51 is set low so that the photoelectric surfaceof the photomultiplier 22 is not saturated by an excessive amount oflight passing through the filer 23. On the other hand, when the amountof light M expected to be emitted from the stimulable phosphor sheet 30is small, the transmittance of the electrochromic element 51 is set highwithin a limit where the photoelectric surface of the photomultiplier 22is not saturated.

[0092] After thus adjusting the transmittance of the electrochromicelement 51, the laser 11 (FIG. 1) is operated. The laser beam L emittedfrom the laser 11 is deflected by the polygonal mirror 12 which isrotated at a high speed in the direction of arrow by the motor 13 and isfocused on the surface of the stimulable phosphor sheet 30 by thescanning lens 14 and is caused to scan the surface of the stimulablephosphor sheet 30 at a constant speed in the direction of arrow X whilethe stimulable phosphor sheet 30 is conveyed in the direction arrow Y.Thus the stimulable phosphor sheet 30 is exposed to the laser beam Lover the entire area thereof.

[0093] The portion of the stimulable phosphor sheet 30 exposed to thelaser beam L emits light M in proportion to the amount of energy storedthereon. The light M emitted from the exposed parts of the stimulablephosphor sheet 30 enters the optical guide 21 together with a part ofthe laser beam L.

[0094] Since the laser beam L is cut by the stimulating light cut filter23, only the light M passes through the filter 23. (See FIG. 2)

[0095] The light M passing through the filter 23 further passes throughthe electrochromic element 51, whose transmittance has been adjustedaccording to the amount of light M expected to be emitted from thestimulable phosphor sheet 30 upon stimulation thereof, and impinges uponthe photomultiplier 22. That is, when the amount of light M isrelatively large, the transmittance of the electrochromic element 51 isreduced and accordingly the amount of light M impinging upon thephotomultiplier 22 cannot be so large that the photoelectric surface ofthe photomultiplier 22 is saturated.

[0096] On the other hand, when the amount of light M is small, thetransmittance of the electrochromic element 51 is set high. In thiscase, however, since the amount of light M emitted from the stimulablephosphor sheet upon stimulation thereof is originally small, there is nofear that the photoelectric surface of the photomultiplier 22 issaturated and the radiation image information can be read out at a highsensitivity.

[0097] The photomultiplier 22 photoelectrically converts the light M toan analog image signal y and outputs the analog image signal y to thelogarithmic amplifier 24.

[0098] The logarithmic amplifier 24 outputs a logarithmic image signal gby logarithmic conversion of the analog image signal y. The logarithmicimage signal g is input into the A/D convertor 25 and is sampled atpredetermined sampling intervals in synchronization with the scanning bythe laser beam L, thereby quantized into a digital image signal S madeup of image signal components for the respective picture elements.

[0099] Thus in the radiation image information read-out system of thisembodiment, the phenomenon of saturation of the photoelectric surfacecan be suppressed and generation of ghost image can be suppressed whileensuring a high sensitivity of the photomultiplier 22.

[0100] A radiation image information read-out system in accordance witha second embodiment of the present invention will be described,hereinbelow. The radiation image information read-out system of thisembodiment mainly differs from that of the first embodiment in that a NDfiler system 54 is used in place of the electrochromic element 51 as thevariable transmittance medium, and accordingly the elements analogous tothose shown in FIGS. 1 and 2 are given the same reference numerals andwill not be described here.

[0101] The ND filter system 54 is disposed between the stimulating lightcut filer 23 and the photomultiplier 22. The transmittance of the NDfilter system 54 to the light M emitted from the stimulable phosphorsheet 30 upon stimulation thereof can be changed continuously orstepwise.

[0102] The radiation image information read-out system of thisembodiment further comprises a transmittance changing means 55 whichchanges the transmittance of the ND filter system 54 by moving thesystem 54 in a direction, shown by the arrow in FIG. 3, perpendicular tothe optical path on the basis of the amount of light M expected to beemitted from the stimulable phosphor sheet 30 determined by the lightamount determination means 53 so that the photoelectric surface of thephotomultiplier 22 is not saturated by an excessive amount of lightimpinging thereupon.

[0103] The operation of the radiation image information read-out systemof this embodiment will be described hereinbelow.

[0104] The radiographing menu for the stimulable phosphor sheet 30 to beread out is first input into the light amount determination means 53.The radiographing menu includes the kind of the object, the material ofthe object, the shape of the object, the size of the object, thedirection of radiographing and the like and the irradiation dose of theradiations to which the object was exposed can be determined accordingto the radiographing menu. The light amount determination means 53determines the amount of radiation energy stored on the stimulablephosphor sheet 30 on the basis of the radiographing menu input andcalculates the amount of light M expected to be emitted from thestimulable phosphor sheet 30 upon stimulation thereof by the laser beamL on the basis of the amount of radiation energy stored on thestimulable phosphor sheet 30 and the stimulating energy of the laserbeam L.

[0105] The radiographing menu may be input into the light amountdetermination means 53 in any one of the manners described above.

[0106] The amount of light M expected to be emitted from the stimulablephosphor sheet 30 determined by the light amount determination means 53is input into the transmittance changing means 55 and the transmittancechanging means 55 changes the transmittance of the ND filter system 54by moving it in the direction of the arrow according to the amount oflight M expected to be emitted from the stimulable phosphor sheet 30.

[0107] That is, when the amount of light M expected to be emitted fromthe stimulable phosphor sheet 30 is large, the transmittance is set lowso that the photoelectric surface of the photomultiplier 22 is notsaturated by an excessive amount of light passing through the filer 23.On the other hand, when the amount of light M expected to be emittedfrom the stimulable phosphor sheet 30 is small, the transmittance is sethigh within a limit where the photoelectric surface of thephotomultiplier 22 is not saturated.

[0108] After thus adjusting the transmittance of the ND filter system54, the laser 11 (FIG. 1) is operated. The laser beam L emitted from thelaser 11 is deflected by the polygonal mirror 12 which is rotated at ahigh speed in the direction of arrow by the motor 13 and is focused onthe surface of the stimulable phosphor sheet 30 by the scanning lens 14and is caused to scan the surface of the stimulable phosphor sheet 30 ata constant speed in the direction of arrow X while the stimulablephosphor sheet 30 is conveyed in the direction arrow Y. Thus thestimulable phosphor sheet 30 is exposed to the laser beam L over theentire area thereof.

[0109] The portion of the stimulable phosphor sheet 30 exposed to thelaser beam L emits light M in proportion to the amount of energy storedthereon. The light M emitted from the exposed parts of the stimulablephosphor sheet 30 enters the optical guide 21 together with a part ofthe laser beam L.

[0110] Since the laser beam L is cut by the stimulating light cut filter23, only the light M passes through the filter 23. (See FIG. 3)

[0111] The light M passing through the filter 23 further passes throughthe ND filter system 54, whose transmittance has been adjusted accordingto the amount of light M expected to be emitted from the stimulablephosphor sheet 30 upon stimulation thereof, and impinges upon thephotomultiplier 22. That is, when the amount of light M is relativelylarge, the transmittance is reduced and accordingly the amount of lightM impinging upon the photomultiplier 22 cannot be so large that thephotoelectric surface of the photomultiplier 22 is saturated. On theother hand, when the amount of light M is small, the transmittance isset high. In this case, however, since the amount of light M emittedfrom the stimulable phosphor sheet upon stimulation thereof isoriginally small, there is no fear that the photoelectric surface of thephotomultiplier 22 is saturated and the radiation image information canbe read out at a high sensitivity.

[0112] The photomultiplier 22 photoelectrically converts the light M toan analog image signal y and outputs the analog image signal y to thelogarithmic amplifier 24.

[0113] The logarithmic amplifier 24 outputs a logarithmic image signal gby logarithmic conversion of the analog image signal y. The logarithmicimage signal g is input into the A/D convertor 25 and is sampled atpredetermined sampling intervals in synchronization with the scanning bythe laser beam L, thereby quantized into a digital image signal S madeup of image signal components for the respective picture elements.

[0114] Thus in the radiation image information read-out system of thisembodiment, the phenomenon of saturation of the photoelectric surfacecan be suppressed and generation of ghost image can be suppressed whileensuring a high sensitivity of the photomultiplier 22.

[0115] A radiation image information read-out system in accordance witha third embodiment of the present invention will be described withreference to FIG. 4, hereinbelow.

[0116] The radiation image information read-out system of thisembodiment mainly differs from that shown in FIG. 1 in that the amountof laser beam L emitted from the laser 11′ is variable. That is, thelaser 11′ is controlled by a light amount determination means 53 whichdetermines the amount of light M expected to be emitted from thestimulable phosphor sheet 30 when the stimulable phosphor sheet 30 isexposed to a given amount of laser beam L and a light amount changingmeans 56 which controls the laser 11′ according to the amount of light Mexpected to be emitted from the stimulable phosphor sheet 30 determinedby the light amount determination means 53 so that the amount of laserbeam L emitted from the laser 11′ is limited to a level such that thelight M emitted from the stimulable phosphor sheet 30 when it isstimulated by the laser beam L cannot saturate the photoelectric surfaceof the photomultiplier 22.

[0117] The light amount determination means 53 may be the same as thatemployed in the first embodiment shown in FIG. 2 and accordingly willnot be described here.

[0118] The operation of the radiation image information read-out systemof this embodiment will be described hereinbelow.

[0119] The radiographing menu for the stimulable phosphor sheet 30 to beread out is first input into the light amount determination means 53.The light amount determination means 53 calculates the amount of light Mexpected to be emitted from the stimulable phosphor sheet 30 uponstimulation thereof by the laser beam L on the basis of theradiographing menu and a given amount of the laser beam L. Here it isassumed that the amount of light M expected to be emitted from thestimulable phosphor sheet 30 when stimulated by the given amount of thelaser beam L is large enough to saturate the photoelectric surface ofthe photomultiplier 22.

[0120] The amount of light M expected to be emitted from the stimulablephosphor sheet 30 determined by the light amount determination means 53is input into the light amount changing means 56 and the light amountchanging means 56 controls the output of the laser 11′ according to theamount of light M expected to be emitted from the stimulable phosphorsheet 30 to reduce the amount of the laser beam L emitted from the laser11′ by a level ΔL. The level ΔL is selected so that the amount of lightM′ expected to be emitted from the stimulable phosphor sheet 30 whenstimulated by the laser beam L′ having an amount of light reduced by theΔL cannot saturate the photoelectric surface of the photomultiplier 22.

[0121] After thus adjusting the output of the laser 11′, the laser 11′is operated. The laser beam L′ emitted from the laser 11′ is deflectedby the polygonal mirror 12 which is rotated at a high speed in thedirection of arrow by the motor 13 and is focused on the surface of thestimulable phosphor sheet 30 by the scanning lens 14 and is caused toscan the surface of the stimulable phosphor sheet 30 at a constant speedin the direction of arrow X while the stimulable phosphor sheet 30 isconveyed in the direction arrow Y. Thus the stimulable phosphor sheet 30is exposed to the laser beam L′ over the entire area thereof.

[0122] The portion of the stimulable phosphor sheet 30 exposed to thelaser beam L′ emits light M′ in proportion to the amount of energystored thereon. The light M′ emitted from the exposed parts of thestimulable phosphor sheet 30 enters the optical guide 21 together with apart of the laser beam L′. Since the laser beam L′ is cut by thestimulating light cut filter 23, only the light M′ passes through thefilter 23.

[0123] The light M′ passing through the filter 23 impinges upon thephotomultiplier 22. Since the amount of the stimulating laser beam L′ isreduced as described above, the amount of light M′ impinging upon thephotomultiplier 22 cannot be so large that the photoelectric surface ofthe photomultiplier 22 is saturated. Accordingly the photomultiplier 22can photoelectrically convert the amount p′ of the light M′ to an analogimage signal y′ without fear that the photoelectric surface thereofbeing saturated and outputs the analog image signal y′ to thelogarithmic amplifier 24.

[0124] The logarithmic amplifier 24 outputs a logarithmic image signalg′ by logarithmic conversion of the analog image signal y′. Thelogarithmic image signal g′ is input into the A/D convertor 25 and issampled at predetermined sampling intervals in synchronization with thescanning by the laser beam L, thereby quantized into a digital imagesignal S′ made up of image signal components for the respective pictureelements.

[0125] Thus in the radiation image information read-out system of thisembodiment, the phenomenon of saturation of the photoelectric surfacecan be suppressed and generation of ghost image can be suppressed whileensuring a high sensitivity of the photomultiplier 22.

[0126] Though, in the radiation image information read-out systems ofthe first to third embodiments, the amount of light M′ expected to beemitted from the stimulable phosphor sheet upon stimulation thereof isdetermined by use of the light amount determination means, the radiationimage information read-out systems need not be limited to such anarrangement provided that the amount of light impinging upon thephotodetector can be controlled not to saturate the photoelectricsurface of the photodetector according to the amount of light to beemitted which may be determined in any manner, e.g., by actuallymeasuring the amount light, by estimation or on the basis of informationinput from the exterior.

[0127] A radiation image information read-out system in accordance witha fourth embodiment of the present invention will be described withreference to FIG. 5, hereinbelow.

[0128] In FIG. 5, the radiation image information readout system of thisembodiment comprises a stimulating light projecting means 110 forscanning a stimulable phosphor sheet 101 with a laser beam A, aphotoelectric convertor means 120 which photoelectrically reads light Bemitted from the stimulable phosphor sheet 101 upon stimulation by thelaser beam A and converts it to an electric signal (an image signal) V0,a signal processing means 130 which carries out a predetermined imageprocessing on the image signal V0, a sensitivity setting means 170 whichsets the sensitivity of the photoelectric convertor means 120, anirradiation energy control means 140 which controls the amount of laserbeam A (irradiation energy) to which the stimulable phosphor sheet 101is exposed per unit area thereof, a preliminary read-out means 150 and aconveyor means 160 which conveys back and forth the stimulable phosphorsheet 101 in the directions of arrows X0 and X1 in perpendicular to thedirection of arrow Y (to be described later).

[0129] The stimulating light projecting means 110 comprises asemiconductor laser pumped solid state laser (SHG) 11 emitting a laserbeam A at a predetermined wavelength, a rotating polygonal mirror 112which deflects the laser beam A, an electric motor 113 which drives thepolygonal mirror 112, an fθ lens 114 which condenses the laser beam Aand a reflecting optical element 115 which changes the direction of thecondensed laser beam A.

[0130] The photoelectric convertor means 120 comprises an optical guide122 which collects light B emitted from the stimulable phosphor sheet101 upon stimulation by the laser beam A and a photomultiplier 124 whichamplifies and converts the light B into an electric signal.

[0131] The signal processing means 130 comprises a logarithmic amplifier132, an A/D convertor 133 and an image processing circuit 134. The imagesignal output from the photoelectric convertor means 120 is subjected toa predetermined image processing by the signal processing means 130 andthen reproduced as a visible image on a display (not shown) or stored ina memory (not shown) for subsequent processing in the form digital imagedata.

[0132] The irradiation energy control means 140 comprises a monitorcircuit 142 which watches the output signal V1 of the logarithmicamplifier 132, a stimulating system control circuit 144 and an AOM(acoustooptic modulator) 146 which intensity-modulates the laser beam Aemitted from the laser 111.

[0133] The operation of the radiation image information read-out systemof this embodiment will be described hereinbelow.

[0134] The stimulable phosphor sheet 101 on which radiation imageinformation has been recorded by, for instance, a radiation imagerecording system (not shown) is placed on the conveyor means 160 andconveyed in the direction of arrow X0 to a read-out section formed bythe stimulating light projection means 110 and the photoelectricconvertor means 120. In the read-out section, the laser beam A emittedfrom the laser 111 travels through the AOM 146 and is focused on thesurface of the stimulable phosphor sheet 101 through the rotatingpolygonal mirror 112, the fθ lens 114 and the reflecting optical element115. The laser beam A is caused to scan the stimulable phosphor sheet101 in the direction of arrow Y (main scanning) by virtue of rotation ofthe polygonal mirror 112 in the direction of arrow K while thestimulable phosphor sheet 101 is conveyed in the sub-scanning direction(the direction of arrow X0 or X1), whereby the stimulable phosphor sheet101 is two-dimensionally scanned by the laser beam A.

[0135] The stimulable phosphor sheet 101 emits light B upon stimulationby the laser beam A and the light B is guided to the photoelectricconvertor means 120 by the optical guide 22. The photoelectric convertormeans 120 converts the light B into an image signal V0 representing theradiation image information stored on the stimulable phosphor sheet 101.

[0136] The method of determining read-out conditions of the radiationimage information read-out system in this embodiment will be describedhereinbelow. The read-out conditions includes various factors whichaffect on the relation between the amount of the light B and the outputof the radiation image information read-out system, e.g., the read-outsensitivity of the photoelectric convertor means 120 which governs therelation between input and output, the scale factor, and the irradiationenergy of the stimulating light. The read-out conditions are determinedso that maximum and minimum amounts S1 and S2 of the light B within therange representing the radiation image information respectivelycorrespond to maximum and minimum signal levels Q1 and Q2 within theinput signal level range in the signal processing means 130, whereby themaximum and minimum amounts S1 and S2 of the light B are reproduced in aproper density range.

[0137] In this embodiment, preliminary read-out is effected prior tofinal read-out. In the preliminary readout, the radiation imageinformation stored on the stimulable phosphor sheet 101 is read out byexposing the stimulable phosphor sheet 101 to a laser beam having energysmaller than the laser beam A employed in the final readout, and theread-out conditions for the final read-out are determined on the basisof the image information obtained by the preliminary read-out throughanalysis to be described later.

[0138] In this particular embodiment, the preliminary read-out iseffected in the same manner as the final readout except that theirradiation energy of the laser beam A is reduced as compared with inthe final read-out. That is, the irradiation intensity of the laser beamA to the stimulable phosphor sheet 101 is weakened byintensity-modulating the laser beam A as emitted from the laser 111 bythe AOM 146, whereby the irradiation energy of the laser beam A per unitarea of the stimulable phosphor sheet 101 is reduced. In this state, thestimulable phosphor sheet 101 is scanned by the laser beam A and light Bemitted from the stimulable phosphor sheet 101 is read out by thephotoelectric convertor means 120 and the image signal V0 thus obtainedis subjected to logarithmic conversion by the logarithmic amplifier 132.Then the logarithmic image signal V1 is converted into a digital imagesignal V2 by the A/D convertor 133 and the digital image signal V2 isinput into the preliminary read-out means 150. The preliminary read-outmay be effected by use of a stimulating light projecting means, aconveyor means and the like provided separately from those for the finalreadout as disclosed, for instance, in Japanese Unexamined PatentPublication No. 4(1992)-1745. Further as the method for reducing theirradiation energy of the laser beam A per unit area of the stimulablephosphor sheet 101, any method may be employed provided that a brief ofthe radiation image information stored on the stimulable phosphor sheet101 can be detected and the read-out conditions can be determined on thebasis of the information obtained.

[0139]FIG. 6 shows an example of the method for determining, in thepreliminary read-out means 150, the read-out conditions (especially theread-out sensitivity) under which the photoelectric convertor means 120reads out the radiation image information in the final read-out. Theimage signal V2 obtained by the preliminary read-out is input into ahistogram making means 154, which makes a histogram for the levels ofthe image signal components for the respective picture elements (i.e., ahistogram for the amounts of light emitted from the respective spots onthe stimulable phosphor sheet 101 exposed to the laser beam A). Thehistogram made by the histogram making means 154 is input into ahistogram analysis means 156 which analyzes the histogram. FIG. 7A showsa pattern of a histogram for a radiation image of the chest. In FIG. 3A,F is the pattern for the mediastinum, G is for the heart, H is for thelung, I is for the skin or a soft part, and J is for the outside of theobject (i.e., a blank portion). By such analysis, a brief of the energystored on the entire stimulable phosphor sheet, e.g., maximum andminimum amounts S1 and S2 of light B in a desired image informationrange (e.g., from the mediastinum to the soft part) can be obtained, andthe read-out sensitivity for the photoelectric convertor means 120 canbe set on the basis of the maximum and minimum amounts S1 and S2 oflight B in the following manner. FIG. 7B shows the histogram shown inFIG. 7A on a graph showing the relation between the amount of light Band the output of the photoelectric convertor means 120. In FIG. 7B, theabscissa represents the amount of light B and the ordinate representsthe output of the photoelectric convertor means 120 in digital values(i.e., the output signal V2 of the A/D convertor 33).

[0140] Thus the maximum and minimum amounts S1 and S2 of light B in adesired image information range are obtained from a histogram made bythe preliminary read-out and the range (Q1 to Q2) of the signal inputinto the signal processing means 130, that is, the range of the signaloutput from the photoelectric convertor means 120, is determinedcorresponding to the values S1 and S2 so that the values S1 and S2 arereproduced properly.

[0141] By changing the scale factor represented by the values S1, S2, Q1and Q2 according to the width of the desired range of the amounts oflight B to be emitted from the stimulable phosphor sheet 101 uponstimulation thereof, the width of the range of the signal levels inputinto the signal processing means 130 can be normally conformed to thewidth of the desired input signal level range (Q1 to Q2). Thereafter theread-out sensitivity is changed so that the position of the range of thesignal levels input into the signal processing means 130 normallyconforms to the position of the desired input signal level range.

[0142] In this particular embodiment, the image signal V0 read out bythe photoelectric convertor means 120 is converted into a logarithmicsignal by the logarithmic amplifier 132 and the logarithmic image signalV1 is converted into the digital signal V2 having components rangingover 10 bits (digital values of 0 to 1023). Then the read-outsensitivity is set so that the range (S1 to S2) of the amount of light Bemitted from the stimulable phosphor sheet 101 conforms to the 10-bitdigital signal V2 and the amount of light (S3 in FIG. 7B) emitted from ablank portion of the radiation image information stored on thestimulable phosphor sheet 101 (e.g., the portion denoted by J in FIG.7A) becomes larger than the amount corresponding to the upper limit ofthe operable range L of the photoelectric convertor means 120 (in thisparticular embodiment, the range of the amounts of light B resulting indigital values of 0 to 1023). The read-out sensitivity can be adjustedby changing the highest voltage HV of the photoelectric convertor means120. Then a control signal V3 is obtained in the manner described aboveon the basis of histogram analysis and the control signal V3 is inputinto the sensitivity setting means 170, which sets the read-outsensitivity of the photoelectric convertor means 120 by changing thehighest voltage HV imparted to the photoelectric convertor means 120 onthe basis of the control signal V3.

[0143] The intensity (irradiation energy) P0 of the laser beam A whenscanning the image area other than the blank portion in the finalread-out (will be referred to as “the steady state”, hereinbelow) may beobtained in advance in the preliminary read-out.

[0144] A method of controlling the intensity of the laser beam A to thestimulable phosphor sheet 101 in the final read-out will be describedwith reference to FIGS. 8A to 8E. After the read-out sensitivity is setin the manner described above, the conveyor means 60 is reversed toreturn (in the direction of arrow X1 in FIG. 5) the stimulable phosphorsheet 101 to the original position (the position before the preliminaryread-out). The intensity of the laser beam B is first set to theintensity P0 in the steady state in the final read-out. The intensity P0may be obtained through analysis of the histogram obtained by thepreliminary read-out, or may be set to a predetermined valuecorresponding to the radiographing menu. In this state, the radiationimage information stored on the stimulable phosphor sheet 101 is readout by the photoelectric convertor means 120 while scanning thestimulable phosphor sheet 101 with the laser beam A.

[0145]FIG. 8A schematically shows a radiation image K0 of an objectstored on a stimulable phosphor sheet 101. K1 denotes a blank portion.FIG. 8B shows change in the amount of light B emitted from thestimulable phosphor sheet 101 when the laser beam A scans the stimulablephosphor sheet 101 along the dashed line in FIG. 8A. FIG. 8C showschange in the irradiation dose or the irradiation energy of the laserbeam A to which the stimulable phosphor sheet 101 is exposed per unitarea thereof. FIG. 8D shows change in the image signal read out by thephotoelectric convertor means 120 (in this embodiment, the output V1 ofthe logarithmic amplifier 132). FIG. 8E shows change in the digitalimage signal V2 obtained by digitizing the image signal V1 by the A/Dconvertor 133. The solid lines in FIGS. 8C and 8D show the correspondingproperties when the control of the stimulating light in accordance withthe present invention is not performed.

[0146] The output signal V1 of the logarithmic amplifier 132 is watchedby the monitor circuit 142, and the stimulating system control circuit144 controls the AOM 146 to reduce the amount of the laser beam A on thebasis of the output of the monitor circuit 142 so that the output signallevel V1 of the logarithmic amplifier 132 when the photoelectricconvertor means 120 detects the light B emitted from the blank portionK1 is minimized in the range higher than the upper limit of the read-outsignal level range of the photoelectric convertor means 120. By changingthe amount of the laser beam A passing through the AOM 146, theirradiation dose to which the stimulable phosphor sheet 101 is exposedper unit area thereof, that is, the stimulating energy of the laser beamA can be reduced. Since the amount of light emitted from the stimulablephosphor sheet 101 upon stimulation by the stimulating light dependsupon the energy of the stimulating light (the laser beam A) per unitarea of the stimulable phosphor sheet 101 as described above, the amountof light emitted from the blank portion K1 can be reduced by reducingthe amount of the laser beam A passing through the AOM 146 when that thelaser beam A is scanning the blank portion K1 is detected through thelevel of the image signal V1 by the stimulating system control circuit144. When such scanning is repeated, the amount of light emitted fromthe blank portion K1 is rapidly reduced. Since the amount of afterglowbecomes smaller as the amount of light emitted from the stimulablephosphor sheet 101 upon stimulation thereof becomes smaller as describedabove, the aforesaid problems caused by the afterglow can be overcomeand the radiation image information can be accurately read out at a highspeed. Further since generation of afterglow itself is prevented byreducing the stimulating light, the S/N ratio can be improved. Thechange in the irradiation energy of the laser beam A to which thestimulable phosphor sheet 101 is exposed per unit area thereof and thechange in the image signal read out by the photoelectric convertor means120 are shown by the dashed line respectively in FIGS. 8C and 8D.

[0147] In the portion KO other than the blank portion, the irradiationenergy of the laser beam A is kept at the value P0 in the steady state,whereby the image signal V1 can be at a signal level suitable for imageinformation required in diagnosis and the radiation image informationcan be reproduced as a visible image at proper gradation.

[0148] In the above description, the irradiation energy of the laserbeam A to which the stimulable phosphor sheet 101 is exposed per unitarea thereof is reduced by the AOM 146, the irradiation energy may becontrolled in various manners. For example, a visible regionsemiconductor laser is employed as the laser 111 and the intensity ofthe visible laser beam may be directly intensity-modulated. Further theirradiation energy may be controlled by changing the scanning speed ofthe laser beam A relative to the stimulable phosphor sheet 101, e.g., bychanging the conveying speed by the conveyor means 160 or the scanningspeed of the laser beam A.

[0149] Further though in the above description, the readout sensitivityof the photoelectric convertor means 120 is set on the basis of theinformation obtained by the preliminary read-out, the preliminaryread-out need not be effected. For example, since a desired read-outsensitivity can be determined in advance according to the radiographingmenu, e.g., whether the object is the chest or the heart, a plurality oflevels of the read-out sensitivity of the photoelectric convertor means120 in relation to different radiographing menus and the read-outsensitivity of the photoelectric convertor means 120 may be set to thesensitivity level corresponding to the radiographing menu for theradiation image information to be read out.

[0150] When erasing residual energy on the stimulable phosphor sheetafter completion of read-out, energy for erasure is sometimes calculatedby carrying out a predetermined signal processing on the read-out signallevel. In such a case, there is a fear that a correct erasing energylevel cannot be obtained when the conventional method of calculation isapplied as it is to the stimulable phosphor sheet which has been readout by the method of the present invention since the signal level forthe blank portion is lowered. On the other hand, since in accordancewith the present invention, the light B emitted from the blank portionK1 is minimized in the range higher than the upper limit of the read-outsignal level range of the photoelectric convertor means 120, the problemthat the read-out signal level for the blank portion exceeds the rangewhere the characteristics of the photoelectric convertor means islinear, which makes it impossible to realize a correct signal level, canbe avoided. Accordingly, by taking into account information on thereduction of the stimulating light during read-out, erasing energy canbe more accurately calculated.

What is claimed is:
 1. A radiation image information read-out method forobtaining an image signal representing radiation image information on anobject stored on a stimulable phosphor sheet by scanning the stimulablephosphor sheet with a stimulating light beam and photoelectricallydetecting light emitted from the stimulable phosphor sheet uponstimulation thereof by a photodetector having a photoelectric surface,wherein the improvement comprises the steps of: inserting a variabletransmittance medium whose transmittance to the light emitted from thestimulable phosphor sheet upon stimulation thereof is variablecontinuously or stepwise into the optical path of the light between thestimulable phosphor sheet and the photoelectric surface of thephotodetector, and changing the transmittance of the variabletransmittance medium according to the amount of light emitted from thestimulable phosphor sheet upon stimulation thereof so that thephotoelectric surface is not saturated by an excessive amount of lightimpinging thereupon.
 2. A radiation image information read-out method asdefined in claim 1 in which the variable transmittance medium is anelectrochromic element.
 3. A radiation image information read-out methodas defined in claim 1 in which the variable transmittance medium is aNCAP type liquid crystal element.
 4. A radiation image informationread-out method as defined in claim 1 in which the variabletransmittance medium is a ND filter system.
 5. A radiation imageinformation read-out method as defined in claim 1 in which the amount oflight emitted from the stimulable phosphor sheet upon stimulationthereof is determined according to the radiographing menu when theradiation image information on the object was recorded on the stimulablephosphor sheet.
 6. A radiation image information read-out method forobtaining an image signal representing radiation image information on anobject stored on a stimulable phosphor sheet by scanning the stimulablephosphor sheet with a stimulating light beam and photoelectricallydetecting light emitted from the stimulable phosphor sheet uponstimulation thereof by a photodetector having a photoelectric surface,wherein the improvement comprises the step of changing the amount of thestimulating light impinging upon the stimulable phosphor sheetcontinuously or stepwise according to the amount of light emitted fromthe stimulable phosphor sheet upon stimulation thereof so that thephotoelectric surface is not saturated by an excessive amount of lightimpinging thereupon.
 7. A radiation image information read-out method asdefined in claim 6 in which the amount of light emitted from thestimulable phosphor sheet upon stimulation thereof is determinedaccording to the radiographing menu when the radiation image informationon the object was recorded on the stimulable phosphor sheet.
 8. Aradiation image information read-out system for obtaining an imagesignal representing radiation image information on an object stored on astimulable phosphor sheet by scanning the stimulable phosphor sheet witha stimulating light beam and photoelectrically detecting light emittedfrom the stimulable phosphor sheet upon stimulation thereof by aphotodetector having a photoelectric surface, wherein the improvementcomprises. a variable transmittance medium which is variablecontinuously or stepwise in transmittance to the light emitted from thestimulable phosphor sheet upon stimulation thereof and is inserted intothe optical path of the light between the stimulable phosphor sheet andthe photoelectric surface of the photodetector, and a transmittancechanging means which changes the transmittance of the variabletransmittance medium according to the amount of light emitted from thestimulable phosphor sheet upon stimulation thereof so that thephotoelectric surface is not saturated by an excessive amount of lightimpinging thereupon.
 9. A radiation image information read-out system asdefined in claim 8 in which the variable transmittance medium is anelectrochromic element.
 10. A radiation image information read-outsystem as defined in claim 8 in which the variable transmittance mediumis a NCAP type liquid crystal element.
 11. A radiation image informationread-out system as defined in claim 8 in which the variabletransmittance medium is a ND filter system.
 12. A radiation imageinformation read-out system as defined in claim 8 in which the amount oflight emitted from the stimulable phosphor sheet upon stimulationthereof is determined according to the radiographing menu when theradiation image information on the object was recorded on the stimulablephosphor sheet.
 13. A radiation image information read-out system forobtaining an image signal representing radiation image information on anobject stored on a stimulable phosphor sheet by scanning the stimulablephosphor sheet with a stimulating light beam and photoelectricallydetecting light emitted from the stimulable phosphor sheet uponstimulation thereof by a photodetector having a photoelectric surface,wherein the improvement comprises a means for changing the amount of thestimulating light impinging upon the stimulable phosphor sheetcontinuously or stepwise according to the amount of light emitted fromthe stimulable phosphor sheet upon stimulation thereof so that thephotoelectric surface is not saturated by an excessive amount of lightimpinging thereupon.
 14. A radiation image information read-out systemas defined in claim 13 in which the amount of light emitted from thestimulable phosphor sheet upon stimulation thereof is determinedaccording to the radiographing menu when the radiation image informationon the object was recorded on the stimulable phosphor sheet.
 15. Aradiation image information read-out method for obtaining an imagesignal representing radiation image information on an object stored on astimulable phosphor sheet by exposing the stimulable phosphor sheet tostimulating light and photoelectrically detecting light emitted from thestimulable phosphor sheet upon stimulation thereof by a photodetector,wherein the improvement comprises the step of setting the sensitivity ofthe photodetector so that the amount of light emitted from a blankportion on the stimulable phosphor sheet becomes larger than thatcorresponding to the upper limit of the operable range of thephotodetector, and controlling the irradiation energy of the stimulatinglight so that the level of a signal component obtained by thephotodetector by reading the light emitted from the blank portion isminimized in the range higher than the upper limit of the read-outsignal level range corresponding to the operable range of thephotodetector.
 16. A radiation image information read-out method asdefined in claim 15 in which preliminary read-out is effected prior tofinal read-out of the radiation image information on the object and thesensitivity of the photodetector is set on the basis of the imageinformation obtained by the preliminary read-out.
 17. A radiation imageinformation read-out method as defined in claim 15 in which a pluralityof levels of the sensitivity of the photodetector is registered inadvance in relation to different radiographing menus and the sensitivityof the photodetector is set to the sensitivity level corresponding tothe radiographing menu for the radiation image information to be readout.
 18. A radiation image information read-out system for obtaining animage signal representing radiation image information on an objectstored on a stimulable phosphor sheet comprising a stimulating lightprojecting means which exposes the stimulable phosphor sheet tostimulating light thereby causing the stimulable phosphor sheet to emitlight in proportion to the amount of energy stored thereon and aphotodetector which detects the light emitted from the stimulablephosphor sheet upon stimulation thereof, wherein the improvementcomprises a sensitivity setting means which sets the sensitivity of thephotodetector so that the amount of light emitted from a blank portionon the stimulable phosphor sheet becomes larger than that correspondingto the upper limit of the operable range of the photodetector, and astimulating energy control means which controls the irradiation energyof the stimulating light so that the level of a signal componentobtained by the photodetector by reading the light emitted from theblank portion is minimized in the range higher than the upper limit ofthe read-out signal level range corresponding to the operable range ofthe photodetector.
 19. A radiation image information read-out system asdefined in claim 18 further comprising a preliminary read-out meanswhich effects preliminary read-out prior to final read-out of theradiation image information and the sensitivity setting means sets thesensitivity of the photodetector on the basis of the image informationobtained by the preliminary read-out.
 20. A radiation image informationread-out system as defined in claim 18 further comprising a means forregistering a plurality of levels of the sensitivity of thephotodetector in relation to different radiographing menus, wherein thesensitivity setting means sets the sensitivity of the photodetector tothe sensitivity level corresponding to the radiographing menu for theradiation image information to be read out.